Exactly what it says 
Please deactivate the following plugins if you wish to continue migrating your site to ClassicPress:
If you keep them active, there might be all sorts of issues from none, to PHP fatal errors, if switching to ClassicPress
You could test on a “testing Site” that uses ClassicPress if any of those plugins keep working on ClassicPress in order to have some time for finding substitutes.
However since their minimal required WordPress version in all cases will be above 5.0, it is very likely that none of those plugins will work fine with ClassicPress, which is why the plugin flags them and asks you to disable those prior to migration
There are a number of solutions you can try to substitute your current plugins, but not all.
Let me find the post we have that marks a few plugins that you might be interested in…
Edit:
This list of plugins will show what you can use instead of those you are using currently.
Be aware that the best would be to use Plugins made specifically for ClassicPress which you can find in the https://directory.classicpress.net directory - however probably you will not find substitutes for what you had active there, at this moment, in said directory, as it is still in the “early days” for it.
- For SEO you might want to look into classic seo: Classic SEO | ClassicPress Plugin
- For calendars you could look into https://simplecalendar.io/
- For all other plugins I do not know any substitute out of the box, but the above linked list could have more solutions, and other members of the Community here could suggest other things
PS:
Can you confirm that with MetaSlider plugin you mean Slider, Gallery, and Carousel by MetaSlider – Image Slider, Video Slider – WordPress plugin | WordPress.org?
That plugin should not be flagged as incompatible since it supports WP below 4.9